Hydraulic actuators are widely used, e.g. in oil and gas industry. In subsea equipment, for instance in a subsea wellhead, a christmas tree, a blow out preventer, a manifold, a tubing string, or a landing string of a subsea oil and/or gas well, hydraulic actuators are frequently used for operating valves, the actuators being located at inaccessible areas. A recurring problem is to monitor the state of such hydraulic subsea actuators, in particular to determine their actuator position. A particular challenge is determining such actuator position without making modifications or adaptations to the hydraulic subsea actuator. In case of a subsea actuator, e.g., a subsea test tree valve actuator, space constraints or other limitations may make it impossible or inconvenient to provide the actuator itself with a position determining device. Also, strict safety constraints and other requirements must be complied with.
Hence, there is a general need in the art for providing a method and an apparatus for determining an estimate of a position of a hydraulic subsea actuator.